
Isn’t that curse is about how the family keep trying to make a true regas so that they can get the secret of regas? But everytime the send someone from their family, the regas will fall in love with the king. Is that why you are saying the marquis was in love?
I think the family sent the regas, a young boy. Not the marquis himself. It happens for generations ago

I was wondering since [SPOILER] when Blondie provoked old guy "you won't find out because you're not a regas" the man looked upset/shocked. The chapter then went on to explain the family's curse, where everyone who tried to become regas ended up in love with the king. so maybe, VERY BIG MAYBE, blondie was poking at one of the old man's past regrets?

No. The Marquess was surprised because the fact that only a real regas could figure out the secret his family kept was, well, supposed to be a secret. It was the only leverage he had over blondie and threatened him with it, so when blondie spilled it his threat was nullified, and now the blondie had the upper hand because he could make it public and make the Marquess lose that power publicly.
Guys guys guys does that mean the old man (duke) liked the previous king?